1. Field of the Invention
The present invention relates to a magneto-optical disk, a recording method for recording information on a magneto-optical disk, and an information reproducing method for reproducing information from a magneto-optical disk.
The recording method and the reproducing method will be referred to overall as a recording and/or reproduction method.
2. Description of the Related Art
In recent years, along with the increase in volume of information, the recording capacity of optical disks has risen and great advancements have been made in digital information processing technology. In digital versatile disks (DVDs), MPEG2 compression technique has been employed—making it possible to record two to four hours of video on one disk. There is no deterioration in quality due to copying of the data in the digital video, therefore when illegal copies are made, high quality copies are illegally produced and the copyright of the video is seriously infringed.
For this reason, in DVDs, a different media ID (disk discrimination information) is recorded for every disk. Namely, the copyright is protected by managing the disk discrimination information in the recording and reproducing apparatus used to restrict illegal copying and restrict reproduction of illegally copied data.
The above-mentioned method of recording the disk discrimination information is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-162031, Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, Japanese Patent No. 3224380, Japanese Patent No. 2771462, etc.
In the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-162031, Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, and Japanese Patent No. 3224380, the disk discrimination information is recorded by forming a stripe-like mark array referred to as a “burst cutting area (BCA)” or a post cutting area (PCA)” at an inner circumference side of the disk.
Japanese Unexamined Patent Publication (Kokai) No. 11-162031 discloses to provide a first recording region and a second recording region on a magnetic film of an optical disk, form a plurality of marks having a magnetic anisotropy smaller than the magnetic anisotropy of the first recording region in a vertical direction to a film surface in the second recording region, and thereby record additional information. In the DVD, the length of the stripe-shaped marks of the BCA is set to 1.3 mm in a disk radial direction. The magnetic film of the mark portion (BCA portion) changes in magnetization state due to the deterioration of the anisotropy of the magnetic film by exposure to a laser beam or reaching the Curie temperature or more.
When reproducing a signal from the BCA formed in this way, a laser beam is focused on the area, the reflected light is divided into two polarization components perpendicular to each other, and an electrical signal corresponding to the magnetization state is extracted from the difference of the intensities (differential detection). The change of the polarization plane of the laser beam striking the magnetic film at the mark portion (Kerr rotation angle) becomes smaller than the Kerr rotation angle of the laser beam striking the magnetic film at a portion other than the mark. Accordingly, additional information is reproduced by the differential detection.
Japanese Unexamined Patent Publication (Kokai) No. 2000-222783 discloses an optical disk having a first recording region, a second recording region, and a guard band region. In the first recording region, the data is recorded by a pattern of pits. In the second region, the disk discrimination information is recorded as a barcode-like mark (PCA). The guard band region is provided between the first recording region and the second recording region. At least the address is recorded in the guard band region.
The PCA of Japanese Unexamined Patent Publication (Kokai) No. 2000-222783 is formed focusing a laser beam on a reflection film of the optical disk constituted by an aluminum layer for laser trimming to thereby form non-reflection portions or a low reflection portions. In the optical disk of Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, both of the data recorded in the first recording region and the disk discrimination information recorded in the second recording region are reproduced by detecting the change of the reflection ratio.
The guard band region of the optical disk of Japanese Unexamined Patent Publication (Kokai) No. 2000-222783 is preferably provided with a width of at least 300 μm. This is set as a width larger than a single jump of the optical head as shown in Japanese Unexamined Patent Publication (Kokai) No. 2000-222783. Further, as shown in Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, control data indicating physical attributes of the optical disk is preferably recorded in the first recording region. When the optical head skips over the control data region from the disk outer circumference side to access the PCA region, however, control of the optical head becomes impossible. In order to prevent this, the optical disk of Japanese Unexamined Patent Publication (Kokai) No. 2000-222783 is provided with the guard band region.
Japanese Patent No. 3224380 discloses a method of recording media discrimination information on optical recording media having a main recording region and a sub recording region. According to this method, a light beam is made to scan a read-in area of the sub recording region to record the media discrimination information. The read-in area is preferably provided in a range of a radius 22.3 mm to 23.5 mm of an optical disk having a diameter of about 120 mm. The BCA of this optical disk is left in an amorphous state in stripe shapes or is left in a crystalline state in stripe shapes. The media discrimation information is recorded by changing the reflection ratio.
As described above, in Japanese Unexamined Patent Publication (Kokai) No. 11-162031, Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, and Japanese Patent No. 3224380, the BCA (or PCA) is formed at the disk inner circumference side.
On the other hand, Japanese Patent No. 2771462 discloses a method of recording the disk discrimination information by applying a magnetic field and focusing a laser beam having a higher power than the case of forming a rewritable mark to form marks with magnetization directions irreversibly changed. In Japanese Patent No. 2771462, the position for recording the disk discrimination information is not limited to the disk inner circumference.
If a higher storage density optical disk can be realized, it will become possible to use a disk having a small-sized diameter for a portable mobile electronic device so as to record a moving picture. For portable mobile electronic device applications, a DVD-RAM disk having a diameter of 80 mm is proposed. In reducing the size of moving picture cameras etc., disks having further smaller diameters are desired.
As an existing small-sized disk, the MiniDisc (MD) having a diameter of 64 mm can be mentioned. The MiniDisc is a magneto-optical disk magnetically recorded with information in a main recording region and reproduced by differential detection. A magneto-optical disk such as a MiniDisc has the advantage that the number of possible rewrites is much larger in comparison with a phase-change type disk like a DVD and also the advantage that an increase of the recording capacity is possible by employing a magnetically induced super resolution (MSR) reproduction system.
Since the MiniDisc has a small-sized diameter, it is suitable for portable mobile electronic device applications. From mainly the viewpoint of the convenience of the user, the next generation of large recording capacity disk recording and reproducing apparatuses will desirably also be able to record and reproduce existing disks. Accordingly, a magneto-optical disk having compatibility with the MiniDisc is a leading candidate as the next generation small-sized disk able to record and reproduce moving pictures.
The existing MiniDisc cannot record and reproduce disk discrimination information, so illegal copying cannot be prevented. Accordingly, in next generation small-sized disks, it is desirable to be able to prohibit illegal copying on the disk side by utilizing disk discrimination information. However, recording and reproducing disk discrimination information by forming a BCA at the inner circumference of a disk by the above conventional method or controlling recording and/or reproduction for preventing illegal copying based on the disk discrimination information would be difficult for a small-sized disk having a diameter of for example about 64 mm.
When recording disk discrimination information by the conventional method, the recording and reproducing characteristics of the user data in the main recording region (first recording region) and the recording and reproducing characteristics of the disk discrimination information in the sub recording region (second recording region) are degraded for the reasons mentioned later when the disk is reduced in size.
If reducing the area of the main recording region, such degradation of the recording and reproducing characteristics can be prevented, but in a disk having the small-sized diameter, it is particularly important to secure a sufficient recording capacity in the limited recording region. If trying to improve the recording and reproducing characteristics in the main recording region and the sub recording region, it no longer becomes possible to achieve the recording capacity required for the recording and reproduction of a moving picture.
As one of the factors degrading the recording and reproducing characteristics of the main recording region, there can be mentioned the fact that the BCA is actually formed meandering on a rough circle.
FIG. 7 is a schematic view of a BCA. The ring-like region between a broken line a and a broken line b is a sub recording region 101. The BCA is the mark array comprised of a plurality of marks 102 long in the disk radial direction arranged in stripe shapes (barcode shape) along the circumferential direction of the disk. A length L of the BCA in the circumferential direction of the disk is not limited to that of the example of FIG. 7 and can be changed in accordance with the disk discrimination information to be recorded.
FIG. 8 is a schematic view enlarging part of FIG. 7 and shows the circumferential direction of the disk (disk rotation direction) on a straight line perpendicular to the disk radial direction. The upper side of FIG. 8 corresponds to the disk outer circumference side of FIG. 7. In FIG. 8, the band-like region between the straight line a and the straight line b is the sub recording region 101. Further, the mark array of the BCA is formed in the band-like region between a curve c and a curve d (hereinafter referred to as a “BCA region 103”). Namely, a plurality of marks 102 of FIG. 7 are formed in the BCA region 103.
According to Japanese Unexamined Patent Publication (Kokai) No. 11-162031 and Japanese Patent No. 3224380, as shown in FIG. 8, a main recording region 104 is formed on the disk outer circumference side of the sub recording region 101 so as to be adjacent to the sub recording region 101. Note that, in the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, the guard band region is formed on the disk outer circumference side of the sub recording region 101 so as to be adjacent to the sub recording region 101.
As shown in FIG. 7, the mark array of the BCA is formed roughly on a circle, but when finely viewing this, it meanders as shown in FIG. 8. As the factor of such meandering, eccentricity of the disk itself can be mentioned. Also, the precision of the disk fixing jig used at the time of forming the marks of the BCA exerts an influence upon the meandering (wobbling).
As disclosed in Japanese Patent No. 3224380, the marks of the BCA are formed by rotating the disk and focusing a pulse-like laser beam on the disk. If focusing the laser beam on the disk in a state where only a focus servo is applied, the focus can be made to track surface wobbling of the disk accompanying rotation of the disk, but the offset of the beam position in the disk radial direction cannot be solved. Accordingly, in actuality, the mark array is not correctly formed on a circle and meanders.
In general, in the BCA, the length in the disk radial direction (distance between the curve c and the curve d) is set to be more than the width of the sub recording region 101 (distance between the straight line a and the straight line b) so that the recording and reproducing characteristics of the disk discrimination information are guaranteed on the sub recording region 101. When the amplitude of the meandering of the mark array is large there is a part at which the sub recording region 101 and the BCA region 103 do not overlap (a circled portion A in FIG. 8), that is, a part in which the disk discrimination information is not recorded in the sub recording region 101, the recording and reproducing characteristics of the disk discrimination information cannot be secured.
In order to prevent this, as shown in FIG. 9, the superimposed margin of the BCA region 103 in the disk radial direction may be made larger. In this case, however, the disk discrimination information interferes with the main recording region 104 adjacent to the sub recording region 101 (refer to the circled portion B of FIG. 9).
In the case of a magneto-optical disk like a MiniDisc in which the information recorded in the main recording region is reproduced by differential detection, when the BCA disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-162031 is formed, both of the recording information of the main recording region and the disk discrimination information will be reproduced by the differential detection. Accordingly, the disk discrimination information signal is overlappingly recorded in the main recording region 104. At the portion B in which these signals interfere, the recording and reproduction of the main recording region cannot be carried out, so the recording capacity of the magneto-optical disk is lowered.
If providing a space between the sub recording region 101 and the main recording region 104 in order to avoid interference of the disk discrimination information with the main recording region 104, the area of the main recording region 104 is reduced. This is disadvantageous for increasing the recording capacity. Slight meandering of the mark array of the BCA and securing the region for avoiding the influence thereof do not become problems in a disk having a relatively large-sized diameter with an extra margin of space. In a small-sized diameter disk, however, the reduction of the recording capacity due to the provision of the space unable to be recorded with information becomes a serious problem.
It can also be considered to make the length of the BCA region 103 in the disk radial direction shorter to secure the recording capacity of the main recording region 104. However, disk discrimination information is usually reproduced in a tracking servo OFF state (tracking off), so if the length of the BCA region 103 in the disk radial direction is shortened, the positioning precision between the magneto-optical disk and the disk recording and reproducing apparatus will become insufficient and it will no longer be possible to correctly reproduce the disk discrimination information. Accordingly, there is a limit to securing the recording capacity by making the length of the BCA in the disk radial direction shorter.
Therefore, it can be considered to record the disk discrimination information while being overlapped on at least one part of the other information, and secure the recording capacity of the main recording region. Japanese Unexamined Patent Publication (Kokai) No. 2000-222783 shows that signal reproduction of the PCA recorded overlapped with pits of the read-in data region is possible. However, the PCA signal is a modulation signal of the reflection ratio, so when the signal based on the pits and the PCA signal become the same in pulse width under certain conditions, the two cannot be discriminated.
Further, Japanese Patent No. 3224380 discloses to record position information etc. concerning the main information in the sub recording region by the pits and record the BCA overlapped at part of the pits. However, the BCA of Japanese Patent No. 3224380 is a portion changed in the reflection ratio, so reproduction of a signal recorded by the pits of the portion where the BCA overlaps is difficult in the same way as the case of the optical disk disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-222783.
As described above, according to the conventional known methods, it is difficult to reproduce other information recorded overlapped with disk discrimination information.
On the other hand, the method of reproducing the disk discrimination information in the tracking servo ON state (tracking on) and making the optical head track meandering of the BCA can be considered. In this case, the overlapping margin of the BCA region 103 with the main recording region 104 can be reduced, so there is a possibility that the disk can be enlarged in recording capacity. However, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-162031 and Japanese Unexamined Patent Publication (Kokai) No. 2000-222783, usually the BCA is formed at the mirror part without grooves at the inner circumference of the disk. In this case, the disk discrimination information cannot be reproduced in the tracking ON state.
In addition to the above problems, in a magneto-optical disk having a high recording density, when trying to reproduce information recorded in the main recording region and the disk discrimination information recorded in the sub recording region by differential detection under the same conditions, there is the problem that a good reproduction signal of the disk discrimination information cannot be obtained.
If employing the MSR reproduction system in a magneto-optical disk, it becomes possible to greatly raise the recording density of the disk and thereby increase the recording capacity of the disk. The MSR reproduction system forms the recording layer by multiple magnetic layers and utilizes the temperature dependency of interchange magnetic coupling or static magnetic coupling between magnetic layers to makes the effective beam diameter smaller.
According to the MSR reproduction system, marks of less than the optically focused beam diameter can be reproduced, so a high resolution is obtained. Up until now, several types of MSR reproduction systems have been proposed (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 6-290496). Magneto-optical disks employing the MSR reproduction system have already been marketed.
MSR is a phenomenon occurring due to the interchange magnetic coupling or the static magnetic coupling between the magnetic layers being cut above a certain temperature. Accordingly, when focusing a laser beam with a power resulting in the magnetic layers constituting the recording layer reaching more than a predetermined temperature, marks having a diameter less than the beam diameter can be reproduced. Such fine marks are used for recording information in for example the main recording region.
On the other hand, the BCA for recording the disk discrimination information is much larger in comparison with the fine marks reproduced by the MSR reproduction system, so it can be sufficiently detected by differential detection not utilizing the MSR reproduction system. The disk discrimination information recorded by the stripe-shaped mark array is reproduced by providing two slice levels in the amplitude level of the reproduction signal and comparing the signals.
When there is little noise component in the reproduction signal, the difference between the two slice levels, that is, the reproduction amplitude margin, is sufficiently large, so good reproducing characteristics are obtained. However, when the mark array of the disk discrimination information is reproduced under conditions where the effect of the MSR appears, the noise of the reproduction signal becomes large and the reproduction amplitude margin is reduced.